The heart of any ultrasound (imaging) system is the transducer device with its transducer elements or transducer cells which convert electrical energy in acoustic energy and back. Traditionally, these transducer elements or transducer cells are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so-called capacitive micro-machined ultrasound transducer (CMUT) devices.
A CMUT cell comprises a membrane (or diaphragm), a cavity underneath the membrane, and electrodes forming a capacitor. For receiving ultrasound waves, ultrasound waves cause the membrane to move or vibrate and the variation and capacitance between the electrodes can be detected. Thereby, the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the membrane to move or vibrate and thereby transmitting ultrasound waves. In other words, when an electrical signal or voltage is applied to the electrodes forming the capacitor, the electric signal or voltage causes the membrane to deflect, thereby creating ultrasound pressure waves. Typically, a CMUT cell is manufactured using microelectronic semiconductor fabrication techniques. The CMUT device offers advantages in terms of frequency coverage and ease of fabrication over contemporary piezoelectric transducer devices. However, CMUT devices currently may still have a disadvantage in terms of the efficiency and acoustic pressure output relative to existing piezoelectric transducer devices.
In an attempt to increase acoustic pressure output, an operating bias voltage applied or supplied between the electrodes can be increased. However, there are limits to the operating bias voltage that can be applied due to dielectric breakdown and charged tunneling effects. There may also be limits to the operating bias voltage due to driving circuitry, for example in the form of an application specific integrated circuit (ASIC). A problem with applying an increased or excessive operating bias voltage can be that the membrane collapses to the substrate and thereby the electrodes may electrically contact each other. In order to separate the electrodes and thereby prevent electrical contact between the electrodes, the CMUT cell can comprise a dielectric layer or dielectric layers between the electrodes. In particular, a first dielectric layer on or as part of the substrate and a second dielectric layer on or as part of the membrane can be used.
It is a currently recognized limitation of CMUT device that if excessive operating bias voltages are applied to the device, the dielectric layers used to separate the electrodes can become more or less permanently charged. This charging effect has been recognized and deemed a problem or “reliability issue” and an undesirable side effect of the construction of the device. In particular, the permanent charge of the dielectric layers reduces the efficiency of the device as an acoustic transducer device. If the output pressure of a CMUT device is measured at a low unipolar bias voltage and if then this bias voltage is increased to a level sufficient to charge the dielectric layers, the CMUT device will show a lower output pressure when driven by the original low bias voltage.
U.S. 20100237807 A1 discloses a system and method for biasing a capacitive ultrasonic transducer (CMUT) device with a circuit that includes a CMUT that includes a first plate and a second plate that form a membrane structure; a circuit voltage source at a complementary metal-oxide-semiconductor (CMOS) compatible voltage; a bias voltage source that applies a bias voltage greater than a CMOS compatible voltage and is applied to the first plate; and readout electronics with an input connected on the second plate side of the circuit. In an embodiment, the bias voltage alternates polarity according to events related to receiving or transmission of a signal. For example, a bias source may be used that alternates polarity periodically during the ultrasound imaging procedure, as opposed to a DC bias source. This may be used to resolve the charging problems that arise while holding CMUTs at constant DC bias.